doc: explain why peersync fingerprinting is sometimes off

solr/core/src/java/org/apache/solr/cloud/SyncStrategy.java

@@ -173,6 +173,9 @@ public class SyncStrategy {
     // if we can't reach a replica for sync, we still consider the overall sync a success
     // TODO: as an assurance, we should still try and tell the sync nodes that we couldn't reach
     // to recover once more?
+    // Fingerprinting here is off because the we currently rely on having at least one of the nodes return "true", and if replicas are out-of-sync
+    // we still need to pick one as leader.  A followup sync from the replica to the new leader (with fingerprinting on) should then fail and
+    // initiate recovery-by-replication.
     PeerSync peerSync = new PeerSync(core, syncWith, core.getUpdateHandler().getUpdateLog().getNumRecordsToKeep(), true, true, peerSyncOnlyWithActive, false);
     return peerSync.sync();
   }